To ensure high availability and operational safety of the electrical power supply and to guarantee personnel safety in the operating area of electrical installations, power supply networks are increasingly used whose active components are separated from the ground potential. In this kind of power supply network, called IT system, an active conductor may exhibit an insulation fault without making it necessary to interrupt the ongoing operation of the installation because no closed circuit can form in this first fault case owing to the ideally infinitely high impedance value between the conductor and ground. In this context, an insulation fault is a faulty state of the IT system that causes the insulation resistance to drop below the admissible insulation level. It becomes clear from this consideration that the resistance against ground (insulation resistance) in the network to be monitored has to be monitored constantly because a potential other fault in another active conductor (second fault) would cause a fault loop and the flowing fault current, in connection with an overcurrent protection device, would result in a shutdown of the installation and in a standstill of operation.
Actively measuring insulation monitoring devices known from the state of the art are connected in the main branch of the IT system between the network conductors and ground via a coupling circuit and superimpose a measuring voltage on the network that is generated by a signal generator and leads to a current flow that is proportional to the insulation resistance. This measuring current causes a voltage drop at a measuring resistance of the insulation monitoring device, said voltage drop being evaluated and leading to a warning message if a pre-settable limit value is exceeded. However, insulation monitoring by means of active insulation monitoring devices as well as by means of insulation monitoring devices that work passively in a switching manner is accompanied by the restriction that only a single insulation monitoring device of this kind can be used per IT system, i.e. per feed point in practice. If multiple insulation monitoring devices of this kind are used per IT system, measured-value registration is disrupted because the devices interfere with one another. On the other hand, if purely passively working insulation monitoring devices are used, not all insulation fault constellations can be registered, in particular not symmetrical insulation losses; hence, these devices can only be used to a limited extent.
Oftentimes, there is the problem that the network sections of a power supply system are supposed to be monitored individually, i.e. in the separated state, yet the network sections can also be electrically interconnected.
Several known solutions exist for this problem:
a. Turning Off Insulation Monitoring Devices
Each network section is monitored in the separated state by its own insulation monitoring device. If network sections are interconnected, the respective redundant insulation monitoring devices are turned off so that only a single device remains online.
This strategy is proved and tested, but it is elaborate because a full-fledged insulation monitoring device is required for each network section that must be capable of completely separating itself from the network when being turned off so that it does not interfere with the measurement of the remaining insulation monitoring device because of its internal resistance. The construction of devices of this kind is elaborate in particular in case of high nominal voltages, which makes their acquisition expensive.
b. Automatic Alternating Operation of the Insulation Monitoring Devices
Each network section is monitored by its own insulation monitoring device. The devices communicate with one another and work in alternating operation so that only a single device is actively coupled to the network at any given time. This method, too, is proved and tested, but it is also accompanied by high costs in acquisition and by the fact that simultaneous monitoring of the separated network sections is impossible without changing the configuration because the devices are constantly in alternating operation irrespective of the switching status of the network.
c. Scanning Systems
An insulation monitoring device is successively coupled to different points of a network by means of a multiplexer circuit. Scanning systems of this kind are difficult to control because, for optimal function, they should be adapted to the measuring method of the respective insulation monitoring device. Additionally, in case of high nominal voltages, increasingly elaborate switching elements are required owing to the necessary voltage differences so that the cost benefit in relation to the previously considered solutions decreases.
d. Additional Use of Passive Insulation Monitoring Devices
An active main insulation monitoring device is used in a central network section. In all other network sections, insulation monitoring devices with a passive measuring method are used.
Passive insulation monitoring devices usually have a simple structure, which makes them cost-effective. It is disadvantageous, however, that symmetrical insulation faults, which often occur because of aging conductors and components, for example, can be detected by the main insulation monitoring device only after the network sections have been interconnected.